The invention concerns screws for injection molding plastic which have wear-resistant surfaces.
Protective surfaces have been used to reduce wear, improve corrosion resistance, and retard erosion. The protective surface must have a hardness that is in balance with the forces acting on the surface. This is particularly true for double screw extruders. In these extruders, the inner surface of the cylinder is exposed to high stresses concentrated in certain areas. Coatings that are too hard in these areas would accelerate the wear of the cylinder and reduce the useful life of the cylinder. Coatings with a relatively high molybdenum content are generally preferred to reduce the localized wear problems.
Armoring layers have been deposited by flame or plasma spraying (with and without subsequent sintering of the deposited layer) and welding. Between the two, welding applies a layer more firmly joined to the base metal than a layer from spraying techniques. Indeed, this inadequate adhesion frequently results in chipping or limited peeling of the protective coating. Such separation may appear after only a few hours of operation and sometimes even due to handling of the screw before installation. Rejection rates of up to 30% are expected for coated screws because the protective layer is inadequately bonded to the base metal.
The layer applied with welding typically comprises alloys of cobalt or nickel containing carbon, tungsten, chromium, boron, silicon in varying proportions. Tungsten carbide may also be added as a hard, wear-resistant layer.